1. Technical Field of the Invention
This invention relates to boots for universal joints used in order to cover and protect an operation portion of the universal joints. More particularly, this invention relates to boots for universal joints having an excellent durability and also having an excellent impact strength. Still more particularly, this invention relates to boots for universal joints made of thermoplastic elastomer.
2. Related Art
Heretofore, it is well known, as shown in FIG. 8, that a boot for a universal joint made of resin is designed to have an annular fitting portion 52 on the side of a relatively large diameter in the axial direction (hereinafter it is called large-diameter side) of a bellows portion 51 and also to have another annular fitting portion 53 on the side of a relatively small diameter in the axial direction (hereinafter it is called small-diameter side) of the bellows portion 51. The fitting portion 52 of the large-diameter side is fixed on a periphery of an outer ring portion 54 of the universal joint, and the fitting portion 53 of the small-diameter side is fixed on an outer periphery of a shaft 55 as shown in FIG. 8, thereby to cover and protect the operation portion of the universal joint. There is an opening portion 60 in the radially inner side of the outer ring portion 54. Bellows usually have several mountainous and ravine shape portions, namely crest 58 and trough 57 portions. Moreover, a frustoconical shape enlarged portion 56 having an approximately frustoconical shape in cross section is provided in the vicinity of the large-diameter-side portion, in order to lower a contact pressure between each adjacent mountainous or slanting portion 59 shape portion at the time when the bellows is bent in a sharp angle, and to also relieve bellows portion 51 from contact with the shaft 55 or the opening portion 60 as well. The enlarged portion 56 is placed between the bellows portion 51 and the fitting portion 52 of the larger-diameter side (Japanese Registered Patent 2525619).
As shown in FIG. 9, the inclination angle between the slanting portions 59 of the bellows extending from the trough portions to the small-diameter side and the axial direction of the boots, is smaller than that of the angle between the enlarged portion 4 exdending from large-diameter side to small-diameter side and the axial direction of the boots. When the boot is bent, the force "f" in the direction of the arrow is added, and the enlarged (frustoconical) portion 56 is forced to move toward the arrow direction "d", to prevent the bellows portion 51 from interfering with the opening portion 60.
Since such kind of boots made of resin are excellent in durability, functionality and recycling ability, when compared to conventional boots made of rubber, so that the resin boots have become widely used in recent years. However, due to the higher stiffness of the resin boots compared to the boots made of rubber, the following inconveniences have resulted.
That is to say, a fitting member is used when the fitting portion 52 of the large-diameter side of the boot made of resin is fixed on the outer ring portion 54 of the universal joint. A clamp is generally used as the fitting member. In this case, the clamp having higher stiffness than that of a conventional clamp for rubber boots has been used, because of the higher stiffness of the resin boots. Caulking type clamps having higher stiffness are usually used for resin boots.
The caulking type clamp is usually provided with a tightly-clamping portion (not shown) in a specific area of its circumference, and the tightly-clamping portion protrudes relatively largely from the outer peripheral surface of the band portion of the clamp, toward the radially outwardly direction.
Therefore, as shown in FIG. 8, crest portion 58 or slanting portion 59 of the first mountainous portion or annular enlarged portion 56 on the large-diameter side of the bellows 51 is prone to bend toward the clamping side and interfere with the tightly-clamping portion or boot's band, when the boot is bent sharply. With this repeating interference, the annular enlarged portion 56 and the bellows portion 51 are prone to be damaged.
There is also a shortage in physical strength against impact in the vicinity of the enlarged portion. When impact is added from outside of the boot in the vicinity of the enlarged portion as shown by the arrow "g" or "g'" in FIG. 9. The portion around the enlarged portion or the slanting portion 59 in FIG. 9 is impinged to the outer ring portion 54, to often cause damage in the boot.
In view of the above-mentioned problems, an object of the present invention is to provide bellows type boots for the universal joint, which does not interfere with a tightly-clamping portion of a boot clamping means, having an excellent durability, and also having an excellent impact strength.